1. Field of the Invention
The present invention relates to a semiconductor device having a circuit constituted by thin-film transistors (hereinafter abbreviated as TFTs) formed on a substrate having an insulating surface and to a method of fabricating the same. More particularly, the invention relates to an electro-optical device as represented by a liquid crystal display device having a pixel portion and a driver circuit formed in the periphery thereof on the same substrate, and technique that can be favorably utilized to electronic equipment mounting the electro-optical device. In this specification, the semiconductor device stands for devices that work by utilizing the semiconductor characteristics, in general, and encompasses the above-mentioned electro-optical device and electronic equipment mounting the electro-optical device.
2. Description of the Related Art
Technique for constituting switching elements and active circuits by using TFTs has been developed in connection with the electro-optical devices as represented by an active matrix-type liquid crystal display device. TFT is the one in which a semiconductor film is formed on a substrate such as of a glass by a vapor-phase growing method and is used as an active layer. As the semiconductor film, there is preferably used silicon or a material comprising chiefly silicon such as silicon-germanium. The semiconductor films can be classified into amorphous silicon films and crystalline silicon films as represented by polycrystalline silicon depending upon the method of formation.
The TFT using the amorphous semiconductor (typically, amorphous silicon) film is not capable of exhibiting an electric field effect mobility of not smaller than several square centimeters per Vsec due to electro-physical properties caused by the amorphous structure. In the liquid crystal display device of the active matrix type, therefore, the switching elements (pixel TFTs) can be formed for driving the liquid crystals in the pixel portion, but the driver circuit cannot be formed for displaying the picture. Therefore, the driver circuit is employing a technique for mounting a driver IC relying upon a TAB (tape automated bonding) system or a COG (chip on glass) system.
On the other hand, the TFT using, as an active layer, a semiconductor film including a crystal structure (hereinafter referred to as crystalline semiconductor) (typically, crystalline silicone or polycrystalline silicone), exhibits a high electric field effect mobility and makes it possible to form circuits of various functions on the same glass substrate and, hence, makes it possible to realize, in addition to pixel TFTs, a shift register circuit in a driver circuit, a level shifter circuit, a buffer circuit and a sampling circuit. Such circuits have been formed relying upon a CMOS circuit constituted by n-channel TFTs and p-channel TFTs. Owing to the above technique for mounting the driver circuits, it is becoming obvious that the TFTs having a crystalline semiconductor layer as an active layer is suited for forming the driver circuits in addition to the pixel portion on the same substrate, from the standpoint of decreasing the weight and thickness of the liquid crystal display device.
From the standpoint of TFT characteristics, it is better to use the crystalline semiconductor layer as an active layer. From the standpoint of fabricating the TFTs suited for various circuits in addition to the pixel TFTs, however, the steps of fabrication become complex and, besides, the number of the steps increase. It is obvious that an increase in the number of the steps drives up the cost of production and, besides, decreases the yield of production.
Further, in order to stabilize the operation of these circuits fabricated by using n-channel TFTs and p-channel TFTs, the threshold voltages and sub-threshold constants (S-values) of the TFTs must be confined within a predetermined range. For this purpose, the TFTs must be studied from both the structure and the materials constituting them.